For read-modify-write data storage systems in digital computers, it is desirable to include a provision for preventing data discovered as bad and uncorrectable during the read process from being rewritten and later read as being correctable data. Such an occurrence is generally the result of the rewritten bad data inadvertently acquiring an additional bit error, which then make certain single bit errors occurring in the stored bad data look like correctable data when reread. The algorithms generally in use for this purpose do not protect the stored bad data from all cases of such single bit error influence.